Casing comprises pipe that is used to line a wellbore and is cemented in place. The casing may extend all the way to the wellhead at the top of the well, or it may extend up only to the lower end of a next upper string of casing. In the latter instance, the casing is typically referred to as a liner. The casing may be installed in a portion of the wellbore that has been previously drilled by drill pipe. Alternately, the casing may itself be used as the drill string to drill portions of the well.
In either event, the individual joints or sections of casing are secured to each other to make up a casing string being lowered into the well. When adding a new joint of casing to a string of casing, the string of casing will be supported by a spider at the rig floor. The spider has a set of slips that support the weight of the casing string. In one technique, the drilling rig has a top drive, which is a rotary power source that travels up and down the drilling rig. A casing gripper is secured to the quill or drive stem of the top drive. The casing gripper has radially moveable gripping elements that will grip either the inner diameter or outer diameter of the joint of casing. A set of links, also called bails, are mounted to the casing gripper to support a casing elevator below the lower end of the casing gripper. The elevator comprises a clamp that fits around the casing joint below the collar on the upper end of the casing joint. Hydraulic cylinders will pivot the bails outward to engage the next joint of casing, which may be spaced laterally from the spider and inclined on a ramp or V-door.
After clamping the elevator around the joint of casing, the driller raises the top drive and allows the links to swing back into vertical alignment with the top of the string of casing. The operator then lowers the top drive and the joint of casing until it lands on and is supported by the string of casing. The operator continues to lower the top drive and the casing gripper while the joint of casing remains supported on top of the string of casing. The gripping elements of the casing gripper will slide into or over the upper end of the joint of casing. Once in place, the operator actuates the casing gripper to grip the joint of casing, then rotates the gripping element to rotate the joint of casing and make it up with the string of casing.
In some instances, the elevator links are quite long because they must be able to pivot laterally outward to engage the next joint of casing as it is supported on the V-door. In large rigs, this lateral distance can be substantial. The operator may be able to adjust the length of the links or use longer links. However, longer links place the elevator several feet below the lower end of the casing gripper. This arrangement makes it difficult for the driller to stab the casing gripper into or over the upper end of the casing, particularly with small diameter casing. The upper end of the casing may be 35 to 40 feet above the driller when the stabbing has to occur, making it difficult to see. Having elevator a considerable distance below the casing gripper results in extra time required for making up a new joint of casing with the casing string.